Organisms are living things. Us humans are also living things, so we are helpful and harmful to other organisms. For example: we can help another human being with something, or we can help a plant/tree grow if we water it. In return, they also help us by providing us with oxygen. however, we can also be harmful to each other and the environment, for example: we can't hurt each other, or plant or animals by killing them or we can hurt insects by stepping on them. Therefore, organisms are helpful and harmful to each other.
A chemical reaction happens when the atoms or molecules rearrange to form a new substance
They discovered that *elements* show increase in atomic numbers across the period.
Answer:
Double replacement reaction
Explanation:
This is what is known as a double replacement reaction. The two parts of each molecule separate and recombine to form two new compounds. For instance, 
separates into 
and 
, while 
separates into 
and 
, and then they recombine with the other compound.
Hope this helps!
